1. TECHNICAL FIELD
The present invention relates to a hard disc type of magnetic disk and its fabrication process.
2. BACKGROUND TECHNIQUE
For magnetic disc drive equipment used with computers, etc., hard types of magnetic discs including a magnetic layer on a rigid substrate have been used and, in recent years, these magnetic discs have been rapidly increased in capacity and decreased in size. These magnetic discs are usually fabricated by the spin-coating of a magnetic coating material, followed by orientation, curing, and so on.
A coated type of magnetic disc thus fabricated by the spin-coating technique has a film that is made thinner on the inner side and thicker on the outer side by centrifugal force.
With a magnetically recorded magnetic disk, on the other hand, recording and reproduction are done at the same frequency. Hence, there is a difference in the relative speed with respect to a magnetic head between the inner and outer sites of the disc, so that the disc can be shorter in recording wavelength and higher in recording density on the inner site than on the outer site. Thus, there arises a problem that the reproduction output becomes low on the inner site and high on the outer site, and this is even true of a magnetic disc having a magnetic film of uniform thickness. Accordingly, such an output difference becomes much larger in the case of a disc that is thinner on the inner side and thicker on the outer site, as is the case with the conventional coated type of magnetic disc.
The higher the relative speed, the larger the amount of levitation of a levitation type magnetic head is and, hence, the farther the disc is from the center, the larger the amount of levitation is. For this reason, the farther the disc is from the center, the more unfavorable the overwriting properties are. In addition, the larger the film thickness, the worse the overwriting properties are. Accordingly, a conventional coated type of magnetic disc having a recording layer that increases in thickness as it is far from the center is further unfavorable in view of the overwriting properties of the outer site.
Thus, a problem with a conventional coated type of magnetic disc is that there is a large difference in reproduction output and overwriting properties between the inner and outer sites.
As magnetic disc size decreases from 14 or 8 inches to 5, 3.5, and 2.5 inches, the inner diameter increases relatively; the ratio between the outer and inner diameters increases, and so the difference in centrifugal force on the outer and inner sites of the disc during spin coating increases, with the resulting increase in the film thickness difference. In addition, there is an increase in the ratio of the relative speeds of the head and disc at the time of record reproduction, which gives rise to a further increase in the differences in terms of reproduction output and overwriting properties.
An approach for solving such problems, for instance, is disclosed in JP-B-61-3545 that is directed to a coating method in which the viscosity of a coating material is controlled by gas spraying depending on where the film is located, followed by spinning-out. The use of this coating method makes it possible to increase the thickness of the film on the inner site by pre-increasing the viscosity of the inner-side film, but film thickness control is difficult at a film thickness of 0.5 mm or below, resulting in a lowering of the squareness ratio of the inner site of the film.
Such problems may also be solved by forming a thicker magnetic layer and then polishing the outer site of the layer to a smaller thickness. In this case, however, the polishing makes film thickness uneven or causes damage to the film at a film thickness of 0.3 .mu.m or below, causing modulation to be increased at the time of recording and reproduction and so often giving rise to errors.